11-1 FRAMING The data link layer needs to pack bits into frames, so that each frame is distinguishable from another. Our postal system practices a type.

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Presentation on theme: "11-1 FRAMING The data link layer needs to pack bits into frames, so that each frame is distinguishable from another. Our postal system practices a type."— Presentation transcript:

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11-1 FRAMING The data link layer needs to pack bits into frames, so that each frame is distinguishable from another. Our postal system practices a type of framing. The simple act of inserting a letter into an envelope separates one piece of information from another; the envelope serves as the delimiter. Fixed-Size Framing Variable-Size Framing Topics discussed in this section:

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11-2 FLOW AND ERROR CONTROL The most important responsibilities of the data link layer are flow control and error control. Collectively, these functions are known as data link control. Flow Control Error Control Topics discussed in this section:

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Flow control refers to a set of procedures used to restrict the amount of data that the sender can send before waiting for acknowledgment. Note

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Error control in the data link layer is based on automatic repeat request, which is the retransmission of data. Note

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11-3 PROTOCOLS Now let us see how the data link layer can combine framing, flow control, and error control to achieve the delivery of data from one node to another. The protocols are normally implemented in software by using one of the common programming languages. To make our discussions language-free, we have written in pseudocode a version of each protocol that concentrates mostly on the procedure instead of delving into the details of language rules.

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11-4 NOISELESS CHANNELS Let us first assume we have an ideal channel in which no frames are lost, duplicated, or corrupted. We introduce two protocols for this type of channel. Simplest Protocol Stop-and-Wait Protocol Topics discussed in this section:

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Figure 11.6 The design of the simplest protocol with no flow or error control

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11-5 NOISY CHANNELS Although the Stop-and-Wait Protocol gives us an idea of how to add flow control to its predecessor, noiseless channels are nonexistent. We discuss three protocols in this section that use error control. Stop-and-Wait Automatic Repeat Request Go-Back-N Automatic Repeat Request Selective Repeat Automatic Repeat Request Topics discussed in this section:

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Error correction in Stop-and-Wait ARQ is done by keeping a copy of the sent frame and retransmitting of the frame when the timer expires. Note

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In Stop-and-Wait ARQ, we use sequence numbers to number the frames. The sequence numbers are based on modulo-2 arithmetic. Note

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In Stop-and-Wait ARQ, the acknowledgment number always announces in modulo-2 arithmetic the sequence number of the next frame expected. Note

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The receive window is an abstract concept defining an imaginary box of size 1 with one single variable R n. The window slides when a correct frame has arrived; sliding occurs one slot at a time. Note